In the world of modern technology, electronic circuits and circuit elements formed on semiconductor chips are ubiquitous. A semiconductor chip may be found controlling a battery-powered toy, a home stereo or a computerized fuel injection system.
Semiconductor chip technology allows circuits to be miniaturized because microscopic circuit elements and electrical connections between those elements can be formed directly on the chip. This process typically includes successively depositing layers of different materials on the semiconductor chip that are needed to form connections and circuit elements. After a single layer or several layers are deposited, much of the deposited material is chemically removed from the chip, leaving material behind only in those places where it is used to form a circuit element or electrical connection.
Because semiconductor chips are mass-produced, when a defect in the layered structure is discovered causing a circuit element on the chip to function improperly or not at all, the cause of the problem must be determined and corrected or it is likely to recur in subsequent chips. Accordingly, a method known as deprocessing is employed.
In deprocessing, the layers of material that have been deposited on the semiconductor chip are chemically or mechanically removed, or chemically undercut and removed, one by one until the area or layer which is suspected of or known to be causing the problem is exposed for inspection. By inspecting the exposed layer with an optical, laser, scanning, electron or other suitable microscope, the cause of the problem can often be visually determined. Microphotographs can also be taken to determine whether the visual defect is indeed the cause of the device failure.
In the prior art, deprocessing is typically performed by placing the chip to be deprocessed in a bath containing a chemical deprocessing agent in liquid form. The chemical deprocessing agent is usually an acid or mixture of acids capable of removing, i.e., dissolving or undercutting, layer(s) of material deposited on the chip. The chip is then removed from the solution for inspection of the exposed, suspect layer.
While deprocessing is a valuable technique in the failure analysis of semiconductor chips, it has at least three principle drawbacks. The first is the amount of time required for deprocessing. It can take hours for the chemical processing to remove the unwanted layers of material.
The second drawback contributes to the first in that, in order to monitor the progress of the deprocessing so that the process is stopped at the layer to be inspected, the deprocessing must be interrupted periodically so that the chip can be inspected and the deprocessing monitored. Otherwise, the layer to be inspected may itself by removed by the deprocessing. This monitoring obviously adds to the time required for deprocessing.
The third drawback is that the chemical removal of layers of material from the semiconductor chip may be uneven. Thus, islands of material from the layer above the suspect or problem layer may remain after deprocessing. This obstructs inspection of the problem layer. Alternatively, such islands of material from the layer above the problem layer, when removed, may take part of the problem layer with them, thereby by ruining the sample.
Accordingly, there is a need for an improved deprocessing apparatus and method that overcomes these drawbacks. Specifically, there is a need for an improved deprocessing apparatus and method that more quickly, accurately, and completely removes unwanted layers of material to expose the layer that needs to be inspected.